`OrderlessPatternSequence[p1, …, pk]` matches the block of `k` arguments at
OrderlessPatternSequence[p1, p2, …]
{MatchQ[{1, 2}, {OrderlessPatternSequence[2, 1]}], MatchQ[{1, 2, 3}, {_, OrderlessPatternSequence[3, 2]}]}
→ {True, True}MatchQ[{1, 2, 3}, {OrderlessPatternSequence[3, 1], _}]
→ False{3, 1, 2} /. {OrderlessPatternSequence[1, x_], ___} :> x
→ 3